1. Field of the Invention
The present invention generally relates to photovoltaic devices. More particularly, it relates to copper indium gallium diselenide/disulfide-based, thin film, photovoltaic devices.
2. Description of the Related Art including Information Disclosed under 37 CFR 1.97 and 1.98
Semiconductor materials like copper indium gallium diselenides and sulfides (Cu(In,Ga)(S,Se)2, herein referred to as “GIGS”) are strong light absorbers and have band gaps that match well with the optimal spectral range for photovoltaic (PV) applications. Furthermore, because these materials have strong absorption coefficients, the active layer in the solar cell is required to be only a few microns thick.
Copper indium diselenide (CuInSe2) is one of the most promising candidates for thin film PV applications due to its unique structural and electrical properties. Its band gap of 1.0 eV is well-matched with the solar spectrum. CuInSe2 solar cells can be made by selenization of CuInS2 films because, during the selenization process, Se replaces S and the substitution creates volume expansion, which reduces void space and reproducibly leads to a high quality, dense CuInSe2 absorber layers. [Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett., 2009, 9, 3060] Assuming complete replacement of S with Se, the resulting lattice volume expansion is ˜14.6%, which is calculated based on the lattice parameters of chalcopyrite (tetragonal) CuInS2 (a=5.52 Å, c=11.12 Å) and CuInSe2 (a=5.78 Å, c=11.62 Å). This means that the CuInS2 nanocrystal film can be easily converted to a predominantly selenide material, by annealing the film in a selenium-rich atmosphere. Therefore, CuInS2 is a promising alternative precursor for producing CuInSe2 or CuIn(S,Se)2 absorber layers.
The theoretical optimum band gap for absorber materials is in the region of 1.2-1.4 eV. By incorporating gallium into CuIn(S,Se)2 thin films, the band gap can be manipulated such that, following selenization, a CuxInyGazSaSeb absorber layer is formed with an optimal band gap for solar absorption.
Conventionally, costly vapor phase or evaporation techniques (for example metalorganic chemical vapor deposition (MO-CVD), radio frequency (RF) sputtering, and flash evaporation) have been used to deposit the CIGS films on a substrate. While these techniques deliver high quality films, they are difficult and expensive to scale to larger-area deposition and higher process throughput. Thus, solution processing of CIGS materials has been explored. One such approach involves depositing CIGS nanoparticles, which can be thermally processed to form a crystalline CIGS layer.
One of the major advantages of using nanoparticles of CIGS is that they can be dispersed in a medium to form an ink that can be printed on a substrate in a similar way to inks in a newspaper-like process. The nanoparticle ink or paste can be deposited using low-cost printing techniques such as spin coating, slit coating and doctor blading. Printable solar cells could replace the standard conventional vacuum-deposited methods of solar cell manufacture because the printing processes, especially when implemented in a roll-to-roll processing framework, enables a much higher throughput.
The synthetic methods developed so far offer limited control over the particle morphology, and particle solubility is usually poor which makes ink formulation difficult.
The challenge is to produce nanoparticles that overall are small, have low melting point, narrow size distribution and incorporate a volatile capping agent, so that they can be dispersed in a medium and the capping agent can be eliminated easily during the film baking process. Another challenge is to avoid the inclusion of impurities, either from synthetic precursors or organic ligands that could compromise the overall efficiency of the final device.
U.S. Publication No. 2009/0139574 [Preparation of Nanoparticle Material, published 4 Jun., 2009], the entire contents of which are incorporated herein by reference, describes the synthesis of colloidal CIGS nanoparticles with a monodisperse size distribution, capped with organic ligands that enable solution processing and can be removed at relatively low temperatures during thermal processing.
One of the challenges associated with the nanoparticle-based CIGS deposition approach is to achieve large grains after thermal processing. Grain sizes on the order of the film thickness are desirable since grain boundaries act as electron-hole recombination centres. Elemental dopants, such as sodium [R. Kimura, T. Mouri, N. Takuhai, T. Nakada, S. Niki, A. Yamada, P. Fons, T. Matsuzawa, K. Takahashi and A. Kunioka, Jpn. J. Appl. Phys., 1999, 38, L899] and antimony, [M. Yuan, D. B. Mitzi, W. Liu, A. J. Kellock, S. J. Chey and V. R. Deline, Chem. Mater., 2010, 22, 285] have been reported to enhance the grain size of CIGS films and thus the power conversion efficiency (PCE) of the resulting devices.
The incorporation of sodium into CIGS is a well-known method for increasing maximum photovoltaic cell efficiencies. The effect of sodium is thought to be an increase in the net carrier concentration and film conductivity, and possibly enhancement of grain growth. Sodium is typically added in concentrations ranging between 0.1-1.0% by weight.
A common method used to incorporate sodium is by diffusion from soda-lime glass (SLG) substrates through a molybdenum back contact layer into an adjacent CIGS layer. The processes limiting or enabling sodium diffusion from the SLG during crystal growth are currently not well understood. One drawback of this method is that the diffusion of sodium is not easily controlled.
Other known incorporation methods include diffusion from a thin sodium-containing precursor layer that is deposited either below or above the CIGS absorber layer, co-evaporation of a sodium compound during the growth of CIGS or soaking the CIGS films into a sodium salt solution. For example, Guo et al. incorporated sodium into films prepared from CIGS nanoparticles by soaking the films in 1 M aqueous sodium chloride solution, prior to selenization. [Q. Guo, G. M. Ford, R. Agrawal and H. Hillhouse, Prog. Photovolt. Res. Appl., 2013, 21, 64]
These methods require either sodium-free substrates or alkali-diffusion barriers (such as Al2O3 or very dense molybdenum). Otherwise, too much sodium may be incorporated into the CIGS if SLG substrates are used.
Examples of sodium compounds typically used in the methods mentioned above include sodium fluoride (NaF), sodium selenide (Na2Se), and sodium sulfide (Na2S).
These incorporation methods involve a multi-step process where the sodium-containing compound is produced at a separate stage, before or after the growth absorber layer. This is achieved by using expensive vacuum deposition techniques and cannot be applied to printable photovoltaic devices produced by printing a CIGS ink on flexible substrates on a roll-to-roll process.
Soaking in a sodium salt solution is a simple method but it is not clear how well the incorporation of sodium may be tuned using this process.
In the prior art, where CIGS films have been prepared by the sputtering of Cu—In—Ga precursors followed by selenization, sodium doping may result in phase segregation of CuInSe2 and CuGaSe2, despite promoting grain growth within the CuInSe2 layer. [F. Hergert, S. Jost, R. Hock, M. Purwins and J. Palm, Thin Solid Films, 2007, 515, 5843] Thus, a nanoparticle-based approach, where the quaternary CIGS phase is inherent within the nanoparticles, may enable sodium-enhanced grain growth without phase segregation.
A method to dope Cu2ZnSnS4 (CZTS) nanoparticles with sodium has previously been described by Zhou et al. [H. Zhou, T.-B. Song, W.-C. Hsu, S. Luo, S. Ye, H.-S. Duan, C. J. Hsu, W. Yang and Y. Yang, J. Am. Chem. Soc., 2013, 135, 15998] The sodium-doped CZTS nanoparticles were prepared by a “hot-injection” method, whereby a sulfur precursor was injected into a solution of copper, zinc and tin precursor salts dissolved in oleylamine at elevated temperature. Following a period of annealing, a solution of sodium trifluoroacetate (CF3COONa) in oleic acid was injected into the CZTS nanoparticle solution, then annealed for a further time period. The ratio of Na/(Cu+Zn+Sn) was tunable from 0.5-10%, and characterization suggested that the sodium was distributed on the nanoparticle surface, rather than homogeneously throughout the nanoparticles. To date, adaptation of the method for the preparation of sodium-doped CIGS nanoparticles has not been reported.
Mitzi and co-workers have explored the incorporation of antimony into CIGS devices formed using a hydrazine solution-based deposition approach. Significant grain growth was observed using Sb2S3/S in hydrazine, with an improvement in PCE from 10.3% for undoped films to 12.3% for films doped with 0.2 mol. % Sb. [M. Yuan, D. B. Mitzi, W. Liu, A. J. Kellock, S. J. Chey and V. R. Deline, Chem. Mater., 2010, 22, 285] At 1.2 mol. %, grain growth could be observed for films annealed at low temperatures (<400° C.). [M. Yuan, D. B. Mitzi, O. Gunawan, A. J. Kellock, S. J. Chey and V. R. Deline, Thin Solid Films, 2010, 519, 852] Despite the improvements in grain size and PCE with antimony doping, the deposition approach carries significant risk owing to the toxic and unstable nature of hydrazine. In addition, the precautions required to safely handle hydrazine pose a significant challenge when scaling the method.
Carrate et al. described ligand exchange process to displace organic ligands on the surface of CZTS nanoparticles, substituting them with an antimony salt (SbCl3), via a biphase system. [A. Carrate, A. Shavel, X. Fontané, J. Montserrat, J. Fan, M. Ibáñez, E. Saucedo, A. Pérez-Rodriguez and A. Cabot, J. Am. Chem. Soc., 2013, 135, 15982] The CZTS-SbCl3 nanoparticles were stable in solution for sufficient time to allow spray deposition.
Though the nanoparticles fabricated by Carrate et al. could be deposited by spray-coating, the lack of organic ligands on the nanoparticle surface may render the material difficult to process using other coating techniques, for which organic components of the ink formulation are critical to its coating properties.
The preparation of antimony-coated CIGS nanoparticles has not yet been reported in the prior art.
Grain growth in CuInSe2 thin films has also been reported upon doping of the CuInSe2 flux with 2 wt. % cadmium or bismuth, followed by localised, pulsed annealing using an electron beam. [R. J. Gupta, D. Bhattacharya and O. N. Sullivan, J. Cryst. Growth, 1988, 87, 151] Grain sizes of up to 10 μm were observed by transition electron microscopy (TEM). However, pulsed annealing may not be an easy process to scale. Additionally, doping with toxic cadmium may be unfavourable.
Thus, a method to form doped CIGS films using a nanoparticle-based deposition approach would be beneficial.